Atherosclerosis is an inflammatory disease of blood vessels resulting in the growth of deposits of fatty substances, cholesterol, cellular waste products, calcium and other substances in the inner lining of an artery. This buildup is called plaque. It usually affects large and medium-sized arteries. Plaques can grow large enough to significantly reduce the blood flow through an artery. But most of the damage occurs when they become fragile and rupture. Plaques that rupture cause blood clots that can block blood flow or break off and travel to another part of the body. If either happens and blocks a blood vessel that feeds the heart, it causes a heart attack. If it blocks a blood vessel that feeds the brain, it causes a stroke.
Atherosclerosis starts when high content of cholesterol in the blood (hypercholesterolemia) leads to the lipid accumulation in the artery wall. This can start even in the fetus if the mother is hypercholesterolemic and progresses during the lifetime. Lipid accumulated in the artery wall undergoes oxidation and attracts inflammatory cells, monocytes, which differentiate into macrophages and take up the lipid. Resulting lipid-loaded macrophage “foam” cells residing in the artery inner layer (intima) are a hallmark of early atherosclerotic lesions. They further contribute to chronic vascular inflammation and the plaque growth. Therefore, studying the mechanisms of macrophage recruitment, lipid uptake and inflammation is important for understanding the pathogenesis of atherosclerosis. A feasible in vivo model of vascular lipid accumulation and inflammation will be also valuable for drug discovery.
Hyperlipidemia and hypercholesterolemia are developed as a consequence of an altered expression of genes regulating lipid metabolism and/or an altered dietary intake of lipids. In mammals, apolipoprotein E (ApoE), apolipoprotein A-I (ApoAI) and low density lipoprotein receptor (LDL-R) are critical proteins that regulate metabolism of lipoproteins, although many other genes are involved.
Current animal models of atherosclerosis: In an effort of establishing animal models of atherosclerosis researchers recreate in animals two major causative factors of the disease in humans, high cholesterol content in blood and high blood pressure. The latter is achieved by administration of high doses of angiotensin. However, the most popular models of atherosclerosis are the ones that achieve high levels of cholesterol in plasma. There are two animal species currently used to model atherosclerosis, hypercholesterolemic rabbits and genetically modified mice. Mouse models are especially widely utilized due to the availability of many transgenic and knockout strains.
Feeding atherosclerosis-susceptible mouse strains (C57BL6) high-fat, high-cholesterol diet causes the development of a very minimal atherosclerotic disease. However, this is greatly accelerated and increased in the mice deficient of either of two genes, which encode the proteins responsible for normal lipid delivery, the low-density lipoprotein receptor (LDLR) and apolipoprotein E (ApoE). The LDLR is on the surface of eventually every cell in the body; it recognizes LDL particles and, via the LDL uptake, the cells get nutrients, including fatty acids and cholesterol. ApoE is a part of lipoprotein particles and it also participates in the cellular uptake of lipid nutrients. The absence of either LDLR or ApoE, combined with the feeding a high-fat, high cholesterol diet, leads to the accumulation of high levels of LDL (the major carrier of cholesterol in blood) in plasma and eventually to its accumulation in artery wall and atherosclerosis.
Thus, LDLR−/− or ApoE−/− mice placed on high-fat diets are currently the two major animal models of atherosclerosis. Although having numerous advantages, these mouse models also have important limitations: A. Relatively long periods of pregnancy, maturation and cholesterol feeding, total from 6 to 12 months; B. A relatively high cost of maintaining of a mouse colony and numerous regulations for the handling of the mammals; C. A complex and labor intensive morphological analysis of atherosclerotic lesions; D. Atherosclerotic lesions in most cases can be analyzed only postmortem, e.g. at one time point only. Existing live animal imaging techniques are inadequate, of low resolution and often use radioactive materials.